Chun Jing,Biqin Dong,Aikif Rz,Tiejun Zhng,Yuxin Zhng,＊

a State Key Laboratory of Mechanical Transmissions,College of Materials Science and Engineering,Chongqing University,Chongqing,400044,PR China

b Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering,Shenzhen University,Shenzhen,518060,PR China

c Department of Mechanical Engineering,Masdar Institute,Khalifa University of Science and Technology,P.O.Box 127788,Abu Dhabi,United Arab Emirates

Keywords:Layered double hydroxide Corrosion inhibition Magnesium alloy Aluminum alloy Steel Concrete

ABSTRACT Layered double hydroxide(LDH),a kind of 2D layered materials,has been recognized as the promising anticorrosion materials for metal and its alloy.The microstructure,physical/chemical properties,usage in corrosion inhibition and inhibition performance of LDH have been studied separately in open literature.However,there is a lack of a complete review to summarize the status of LDH technology and the potential R&D opportunities in the field of corrosion inhibition.In addition,the challenges for LDH in corrosion inhibition of metal-based system have not been summarized systematically.Herein,we review recent advances in the rational design of LDH for corrosion inhibition of metal-based system(i.e.Mg alloy,Al alloy,steel and concrete)and high-throughput anticorrosion materials development.By evaluating the physical/chemical properties,usage in metal-based system and the corrosion inhibition mechanism of LDH,we highlight several important factors of LDH for anticorrosion performance and common features of LDH in applying different metal alloys.Finally,we provide our perspective and recommendation in this field,including high-throughput techiniques for combinatorial compositional design and rapid synthesis of anticorrosion alloys,with the goal of accelerating the development and application of LDH in corrosion inhibition of metal-based system.

1.Introduction

Metals alloys such as Mg alloy,Al alloy,steel are indispensable components of the modern society and involve in all fields of the global economy due to its numerous merits such as high strength,excellent machinability and malleability[1-3].However,due to the diversity and complexity of the application environment,especially in sea water environment,metals alloys usually suffer from the corrosion reaction which destroy the stability and durability,and cause security risks and economic losses[4].According to the recent reported by Hou et al.,the cost of corrosion in China in 2014 was RMB 2.1 trillion,accounting for about 3.34% of GDP[5].It is quite a staggering number,equivalent to 1555 yuan per citizen per year in corrosion cost.The essence of metal corrosion is the oxidation process in which metal atoms lose electrons under the induction of the external environment.As a result,a large number of useless corrosion products are produced,which reduces the structural stability and durability of metal system.The degree of corrosion can be analyzed qualitatively or quantitatively by analyzing the type and content of oxidized products.How to restrain oxidation process is the key to inhibit metal corrosion.At present,isolate the corrosion solution and metal substrate by physical or chemical methods is the main strategy of corrosion inhibition.For example,corrosion inhibitors can adsorb on the surface of metal alloy to form an insoluble molecular film,thus inhibiting the metal corrosion[6].The organic coating acts as a waterproof material to prevent the contact of corrosion solution with metal [7].The galvanized layer protects carbon steel from corrosion by self-sacrifice method[8].Graphene can be used as a physical barrier to protect galvanized steel[9].These successful cases promoted the development of anti-corrosion methods.

Layered double hydroxides(LDH)is a type of layered synthetic anionic clays with an easily tunable brucite structure[10].LDH consists of two components,one is composed by divalent and trivalent cations and named as the host layer or cation layer,the other is composed by organic or inorganic anions and named as the anion layer.The charge of cation layer is balanced by the anion layer.Hence,the entire LDH materials is electroneutral.The interlayer anions of LDH can be exchanged by physical or chemical methods.Hydrotalcite is earliest discovered LDH in nature.Hydrotalcite is composed by divalent Mg2+,trivalent Al3+and CO32-[11].With the development of preparation methods,many novel kinds of LDH,such as LiAl LDH[12],ZnAl LDH[13],NiAl LDH[14],NiFe LDH[15],have appeared.Based on the exchangeability of interlayer ions,anionic-type corrosion inhibitors can be load to the gallery of LDH to inhibit the corrosion of metal alloys[16].In addition,LDH-based physical protective film can be grown on the surface of metal alloy to prevent corrosion[17].The cases confirm that LDH possesses outstanding corrosion inhibition potential.Up to date,there are many research papers on corrosion inhibition of LDH.However,review papers about corrosion inhibition of LDH are so rare.Guo et al.[18]summarized the preparation and application of LDH coating on Mg alloy.Iqbal et al.[19]summarized the preparation method and conditions of LDH-based smart coating on Al alloy substrate.Unfortunately,there is no review about the application of LDH in different metal system such as Al alloy,steel and concrete.

Fig.1.The idealized structure of -intercalated LDHs with different M2+/M3+molar ratios showing the metal hydroxide octahedra stacked along the crystallographic c-axis.

Herein,we summarized the corrosion inhibition of LDH in metalbased system.Firstly,the corrosion inhibition mechanism of LDH was described by considering the microstructure of LDH.Secondly,the preparation method of LDH and the key factors affecting the corrosion inhibition performance were analyzed.Finally,the application of LDH in Mg alloy,Al alloy,steel and concrete were summarized in detail.On the basis of the above,we present our perspective and outlook for futher development of LDH in corrosion inhibition.We hope to promote the industrial corrosion application of LDH through this review.

2.Structural characteristic and corrosion inhibition mechanism of LDH

2.1.Structural characteristic

The crystal structure of LDH is related to the physical or chemical properties.A insightful understanding of the crystal structure is helpful to optimize the macroscopic properties of LDH.At present,there are many reports on the crystal structure of LDH,the most accepted crystal structure form of the layered LDH iswherein M2+and M3+represent the divalent(Mg2+,Zn2+,Ni2+,Co2+etc.)and trivalent metal cations(Al3+,Co3+,Fe3+,Mn3+etc.)located in host layers,OH represents the hydroxyl group,An-represents organic or inorganic anions in the guest layers such as2-mercaptobenzothiazole anions,organic phthalates,a variable parameter of x equals M2+/(M2++M3+)and is in the range of 0.2-0.33,n represents the valence of the intercalated anions[20].The theoretical structure model of-intercalated LDH is shown in Fig.1.The OH-group surrounds the M2+in a coordination form of octahedral unit so as to form a M2+(OH)6structure.Because each-OH group is shared by three adjacent octahedral units,the octahedron unit of the M2+(OH)6is the M2+(OH)2in nature.The M3+replaces the M2+in the octahedral unit partially,resulting in an increased charge.The increased charge is related to the substitution ratio(x)and balanced by the anions in the gallery of LDH.The number of anions equals x/n which represents the charge ratio between the cations layer and anions.The cations in the host layer extend indefinitely along the a and b axes to form the layered structures,and the anions in the guest layer enter the gallery to compensate for the charge imbalance in the host layer.The size of anion affects the layer spacing of LDH.The anions in the gallery of LDH can be exchanged by chemical modification.As a result,the layer spacing of LDH can be regulated.The interaction between the anion in the gallery and the host layer affects the exchange process of anions.According to the previous reports,the anion exchange equilibrium constants followed the sequence:addition,the charge number and ion radius of the cations in the host layer also affect the layer spacing.Therefore,it is feasible to regulate the microscopic crystal structure of LDH according to the requirement of macroscopic properties.It is worthy mentioning that,univalent metal ions(Li+)or tetravalent metal ions(Ti4+or Zr4+)have been successfully incorporated into the main layer of LDH,which is a big step for the diversification of LDH.

2.2.Corrosion inhibition mechanism

LDH is widely used for corrosion inhibition of various metals and their alloys.However,LDH is only suitable for corrosion inhibition in the neutral(NaCl solution)or alkaline systems(simulated concrete pore solution with NaCl),because LDH is layered double hydroxide in nature which has a clear tendency to dissolve in acidic systems.The corrosion inhibition performance of LDH mainly comes from its structural and chemical properties,including physical protective film,nanocontainers of corrosion inhibitors chloride ions binding and self-healing effect.These characteristics enhance the corrosion inhibition of LDH for metal and its alloy.

Fig.2.(a)The SEM images of anodic oxidation layer,(b)the cross-sectional SEM images of anodic oxidation layer,(c)the SEM images of MgAl LDH layer,(d)the cross-sectional SEM images of MgAl LDH layer,(e)the EDS mapping of MgAl LDH layer[28].(f,g)The SEM images of intercalated MgAl LDH at different magnification,(h,i)the cross-sectional SEM images of intercalated MgAl LDH at different magnification[30].

2.2.1.Chloride ions binding

Is well-known that penetration and erosion of chloride ion(Cl-)are the main effect factors of metal corrosion.Finding a reasonable method to weaken the penetration of Cl-is the effective strategy for inhibiting metal corrosion.Cl-can be adsorbed into the gallery of LDH by the anion exchange process.This phenomenon is named as the Cl-binding effect.Cl-binding effect enhances the application value of LDH in the anticorrosion field.Before analyzing the binding ability of Cl-,it is necessary to understand the anion exchange behavior of LDH from the perspective of thermodynamics and kinetics.Recently,Zhao et al.[23]used molecular dynamics and DFT study to determine the relative binding energy of different anions and the Gibbs free energy changes of the anion exchange reactions in LDHs.According the simulated result,the values of relative binding energy obey the order asanions>divalent anions>monovalent anions.Hence,the binding energy is mainly determined by the carried charges of the anions.For the anions with the same charges,the relative binding energy is good linearly correlated with the interlayer spacing.In other word,increasing the layer spacing is helpful to increase the anion exchange capacity of LDH.Due to the large anion radius,the organic anions are not easy to be introduced into the gallery of LDH.Hence,anion radius also affects the anion exchange process.Zhao et al.obtained an inference that when the cell parameter c<24.0?(d003<8),the anion exchange order is mainly determined by the thermodynamic factor.Whereas,when the cellparameter c>24.0?(d003>8?),both the thermodynamic and the dynamic factors affect the anion exchange.In general,using the easy exchanged anion intercalated LDH is helpful to enhance the binding ability of chloride ions.

To evaluate the anion practical effect for Cl-adsorption,Xu et al.[24]preparedand p-aminobenzoate intercalated MgAl LDH by co-precipitation method and then evaluated the Cl-adsorption capacity.The Cl-adsorption capacity of these samples obeys the ordersintercalated MgAl LDH(2.853 mmol g-1)>intercalated MgAl LDH(2.813 mmol g-1)>p-aminobenzoate intercalated MgAl LDH(1.533 mmol g-1).Theintercalated MgAl LDH exhibits maximum Cl-adsorption capacity due to the low affinity and ion radius ofand large exchange dynamics betweenand Cl-.The characteristic peaks of(003)plane ofandintercalated MgAl LDH are very similar due to the similar anion radius.However,the characteristic peak of(003)plane of p-aminobenzoate intercalated MgAl LDH moves to the small diffraction angle direction obviously.The reason should be ascribed that,the large anion radius of p-aminobenzoate.p-aminobenzoate needs to greater layer spacing thanto meet the demands of intercalation.Hence,the variation of diffraction angles of(003)and(006)plane is helpful to judge whether the anion exchange process occurs.Chen et al.[25]preparedintercalated ZnAl LDH to adsorb the Cl-in pure NaCl solution and simulated concrete pore solution(Ca(OH)2,KOH,NaOH,Ca(HCO3)2and NaCl).The maximum Cl-adsorption capacity of ZnAl-NO3LDH in simulated concrete pore solution achieves 3.761 mmol g-1which is slightly lower than LDH in NaCl solution(4.177 mmol g-1).The reason should be ascribed to the competitive adsorption betweenand Cl-.has a high affinity with the host layer of LDH,thereby occupying the active site of LDH and inhibiting the adsorption of Cl-.

Ke et al.[26]used annealed MgAl LDH as the binding agent of Cl-and carbonatein simulated pore solutions with a varied concentration ratio of[Cl-]/[OH-].The adsorption process of annealed MgAl LDH can be divided into two steps:

wherein(a-2)+b+2c=x.First,the MgAl oxides undergoes hydration to achieve crystalline transformation to obtain a layered MgAl LDH(Eq.(1)).This phenomenon is summarized as the memory effect of LDH.Second,the anions in the solution such as Cl-,OH-,are bound in the interlayer of MgAl LDH(Eq.(2)).In the presence ofthe maximum adsorption capacity of annealed MgAl LDH for Cl-achieves about 250 mg g-1which is higher than that of annealed MgAl LDH in the absence of(130 mg g-1).The reason should be ascribed to the competition betweenand Cl-.Hu et al.[27]prepared the MgAl-LDH nanoparticles by the hydration of MgO with Al2O3in the concrete.They found that adding 2%LDH into concrete is conductive to reduce 30%diffusion efficiency of Cl-without obvious strength decline.

2.2.2.Physical protective film

As is well-known,in NaCl-containing environment,the interface interaction of Cl-and metal substate is a necessary condition for the occurrence of metal corrosion.Hence,to inhibit the metal corrosion,formation of the protective film on metal substrate to isolate the Cl-is a feasible method.As an inorganic 2D nanomaterial,LDH can form a physical protective film by means of dense accumulation.For example,Zhang et al.[28]prepared MgAl LDH physical protective film by the hydrothermal conversion of anodic layer on AZ31 Mg alloy.The existence of anodic oxidation layer enhances the structure stability and adhesion of LDH layer.In addition,the pores and cracks are sealed by the LDH and anodic oxidation layer,resulting in a sealed pathway for the aggression of Cl-.In addition,the characteristic treatment of boiling water sealing in this paper reduces the porosity of anodic oxidation layer which is also faborable to the corrosion inhibition.From Fig.2a,the obtained anodic oxidation layer exhibits a uniform and flat morphology and low porosity due to the secondary sealing effect by boiling water treatment.The thickness of anodic oxidation layer is about 1.0μm(Fig.2b).After the hydrothermal conversion is conducted,the MgAl LDH nanosheets is obtained(Fig.2c).The thickness of physical protective film increases to 1.9 nm due to the growth of MgAl LDH(Fig.2d).The EDS mapping shows the main elements of MgAl LDH layer are Mg,O,Al,confirming the success of the preparation process(Fig.2e).Similarly,Mata et al.[29]fabricated LiAl-LDH layer on AA2024 Al alloys by the conversion of anodic oxidation layer.About 3μm anodic oxidation layer is obtained by the treatment of tartaric-sulfuric bath.The LiAl-LDH conversion film is obtained at the alkalic system(NaOH and NaNO3)using LiNO3as the additional metal salt.Due to the synergistic sealing effect of inner anodic oxidation layer and the outer LDH layer,the treatment technique exhibits a superior anticorrosion performance.

Fig.5.The synthesis methods of LDH in corrosion inhibition field.

Although LDH phase is obtained by in-situ transformation of anodic oxidation layer,the complex preparation condition and crystal components in obtained physical protective film limit the understanding of specific role of LDH in anti-corrosion process.Zeng et al.[30]fabricated the physical protective film on AZ31 Mg alloy by combining the coprecipitation and hydrothermal process.The obtained physical protective film composed of pure MoO42-intercalated MgAl LDH.LDH power,which prepared in isolation,is difficult to stabilize on the surface of Mg alloy.Hence,a long-time aging process is needed to form a dense and stable protective film.The obtained dimensions ofintercalated MgAl LDH are about 2-4 nm,and the thickness of protective film is about 17μm(Fig.2f and g).From the sectional view,the protective film is constructed by the compact-packed MgAl LDH nanosheets(Fig.2h and i).Iqbal et al.[31]reported an in-situ growth of MgAl LDH on AA6082 Al alloy.The effect of growth temperature on LDH is evaluated.The dissolution of Al3+from AA2024 provides the abundant Al3+sources.Hence,to achieve the crystal growth of MgAl LDH,the precursor solution just requires single metal cation(Mg2+).According to the dissolution of Al3+in Al alloy,Xie et al.[32]constructed an Al-containing layer on the surface of Q235 steel.The Al-containing layer turn into the ZnAl LDH in a Zn2+-containing solution after the alkalic hydrothermal process.Subsequently,the ZnAl LDH is intercalated within the Na2MoO4solution via an anion exchange process.The obtained film exhibits an outstanding corrosion inhibition performance due to the dense ZnAl LDH film which inhibits the aggression of Cl-.Of course,as an inorganic corrosion inhibitor,theexhibits a superior anticorrosion performance.In general,the construction of physical protective film of LDH can achieve the effective corrosion inhibition for Cl-.To date,there are two methods to construct the physical protective film of LDH:(1)in-situ conversion of anodic oxide to LDH layer,(2)in-situ or ex-situ growth LDH on the surface of alloy by hydrothermal or coprecipitation method.We need to choose suitable methods according to different environmental conditions and application scenarios.In overall,Mg alloy and Al alloy are more suitable for preparing the physical protective film than steel.

2.2.3.Nanocontainer of corrosion inhibitor

Fig.6.The structural transformations of CuZnAl-LDH[66].

Fig.7.The SEM images of the fabricated MgAl LDH film by conversing anodic film with different hydrothermal times(a,b)0 min,(c,d)10 min,(e,f)30 min,(g,h)1 h,(i,j)2 h,(k,l)3 h,(m,n)6 h and(o,p)12 h[44].

Fig.8.The SEM images of(a)abraded AZ31 substrate,(b)Zn-Al alloyed surface,(c)MgZnAl LDH film and(d)superhydrophobic LDH-LA film;(e)Schematic illustration of the formation of the Zn-Al alloyed surface layer,LDH film and superhydrophobic LDH-LA film[80].

As mentioned above,inherent anions in gallery of LDH can be exchanged by other extraneous anions by physical/chemical modification.Based on this feature,some unique anions can be used to intercalate LDH via an anion exchange process,and then released it slowly in specific system via a slow-release effect.Corrosion inhibitor is widely used owing to its outstanding corrosion inhibition efficiency and low use cost[33,34].Hence,by combining the character of LDH with inhibitor,the long-term corrosion inhibition by using anion-type inhibitors intercalated LDH can be realized.These materials are named as the nanocontainer of corrosion inhibitor.So far,there have been many reports about LDH as a nanocontainer.For example,Liu et al.[35]prepared 5-Methyl-1,3,4-thiadiazole-2-thiol(MMT-)intercalated ZnAl LDH bythe exfoliation of ZnAl-LDH in formamide and then reassembly in MMT-aqueous solution(LDH-MTT-(R)),as shown in Fig.3a.Due to the presence of MMT-in the gallery of ZnAl LDH,the characteristic peak of(003)plane moves to the small diffraction angle(Fig.3b).In addition,the loading of MMT-leads to the appearance of characteristic UV-vis adsorption peak at 288 nm(Fig.3c)and N1sand S2pof XPS spectra.The anion release kinetics of LDH-MTT-(R)(Fig.3d)reveals that the anion concentration increases rapidly within 0-10 h and reaches about 1.15 mmol L-1with the increase of immersed time.Subsequently,the anion concentration increases with a slowing growth rate within 10-50 h and reaches about 1.4 mmol L-1.Afterwards,the anion concentration maintains stable and reaches a dynamical release equilibrium.The XRD pattern of LDH-MTT-(R)(Fig.3e)indicates LDH is intercalated by the Clin NaCl solution due to the presence of(003)plane with an interlayer spacing of 0.775 nm.Hence,the dynamical release equilibrium comes from complete anion exchange between MMT-and Cl-.The corrosion tests exhibit that the sample have an outstanding corrosion inhibition efficiency and long-term corrosion inhibition.Hence,LDH in this paper not only plays the role of nanocontainer of corrosion inhibitor,but also binds Cl-during the anion exchange process.

Fig.9.The salt spray test of(a)silane coating,(b)ZnAl-NO3 LDH/silane coating and(c)ZnAl-PO4 LDH/silane coating after 600 h and 1000 h[36].

Alibakhshi et al.[36]prepared ZnAl-LDH and ZnAl-LDH.The ZnAl-LDH is prepared by one-step coprecipitation method,and the ZnAl-LDH is prepared by a direct anion exchange process betweenand.Therelease curve exhibits a rapid release ofat the initial 24 h and then trend to release kinetic equilibrium gradually.The Rctvalue of mild steel achieves maximum after immersing in 3.5 wt% NaCl solution with-containing solution for 24 h.The result confirms that the release offrom LDH and formation of theprotective film on the surface of mild steel needed plenty of time.In general,ZnAl LDH as ananocontainers is a high-efficient corrosion inhibition material for mild steel.Based on this property,the ZnAl-LDH nanocontainers are added into silane coating.The corrosion inhibition performance can further enhance due to the superior physical protective role of epoxy coating.Anjum et al.[37]prepared Mg-Al LDH on AZ31 Mg alloy with the intercalation of 8-hydroxyquinoline(8HQ)by in-situ hydrothermal method with different addition 8HQ dose.As the corrosion inhibitor of AZ31 Mg alloys,the 8HQ exhibits an outstanding corrosion inhibition due to the strong complexation with the dissolved Mg2+and Al3+.The obtained products form a dense and stable insoluble layer on AZ31 surface to block the corrosion active sites.The 8HQ intercalated MgAl LDH has about 2.4 times anti-corrosion efficiency ofintercalated MgAl LDH.The result is due to the synergistic effect of corrosion inhibition of 8HQ inhibitor and the in-situ physical protective film of MgAl LDH.Besides,the Cl-binding of MgAl LDH is also favorable for the corrosion inhibition of AZ31 Mg alloy.Serdechnova et al.[38]fabricated NO3and VOxintercalated ZnAl LDH film by converting the plasma electrolytic oxidation layer of AA2024-T3 Al alloy.The VOxintercalated LDH film exhibits a more better corrosion inhibition performance than that of NO3intercalated LDH film.Similarly,Tang et al.[39]prepared ZnAl LDH physical protective film with varying anion intercalation on AZ31 Mg alloy.The corrosion inhibition capability of the ZnAl LDH with different anion intercalation is sorted as below:ZnAl---LDHs>ZnAl--LDHs>ZnAl--LDHs>ZnAl-Cl--LDHs>ZnAl--LDHs,where ZnAl-LDHs exhibits the best corrosion inhibition performance because of the large interlayer spacing to promote the anion release and Cl-binding and excellent corrosion inhibition of

Fig.10.Schematic illustration of the LDH-containing superhydrophobic stainless steel mesh[94].

Fig.11.Schematic illustration and SEM images of LDH,U-PDMS/-LDH and superhydrophobic coating.

Fig.12.The application of LDH in Mg alloy protection.

In general,the LDH as the nanocontainer of inhibitor has great potential in a variety of corrosion application.

2.2.4.Self-healing

Currently,formation of an insoluble physical or chemical protective film on the metal surface is the main strategy to inhibit metal corrosion.However,the protective film suffers from inevitable damage due to external force in the practical applications.The damaged protective film cannot play the role in corrosion inhibition,instead it would accelerate the corrosion reaction.To solve this problem,it is particularly important to develop a self-healing corrosion inhibition method.Recently,there are many reports on the self-healing effect of LDH nanocontainer.Even when the protective layer is damaged,the release of inhibitor ions can regenerate the insoluble protective film on the breakage of protective layer through the complexation between inhibitor anion and dissolved metal ions.For example,Chen et al.[40]prepared aspartic acid(ASP)intercalated MgAl LDH(MgAl-ASP-LDH)on AZ31 Mg alloy.For comparison,theintercalated MgAl LDH(MgAl--LDH)is prepared by similar method without using ASP.The MgAl-ASP-LDH exhibits a higher corrosion inhibition efficiency than pure ASP and MgAl--LDH due to the coexisting physical protective film of LDH and corrosion inhibition of ASP inhibitor.According to the scratching experiment of AZ31 Mg alloy,obvious self-healing effect on the damage region can be found after AZ31 Mg alloy immersed in 3.5 wt% NaCl solution with for 20 d.The ASP anion adsorbs on damage region of Mg alloy rapidly and then hinders the further corrosion.The self-healing ability enhances the corrosion resistance of magnesium alloy greatly,and endows the long-term stability of Mg alloy in the Cl-containing environment.Yan et al.[41]prepared methionine intercalated MgAl LDH(Met-LDHs)andintercalated MgAl LDHAs a nanocontainer of Met inhibitor,the Met-LDHs exhibits a higher corrosion inhibition than-LDHs due to the outstanding corrosion inhibition of Met.The authors added the Met-LDHs into epoxy coating and spayed on the surface of Mg alloy.The epoxy resin binds the Met-LDHs in the interior of coating,preventing the rapid release of Met inhibitor from the gallery of LDH to corrosion medium.Therefore,the long-term self-healing effect of Met inhibitor is achieved.Jiang et al.[42]prepared PEO coating(plasma electrolytic oxidation)on AZ91 Mg alloy at first and then coated theintercalated MgAl-LDH film(PEO-LDH),as shown in Fig.4a.The obtained specimen is treated by 1H,1H,2H,2H-perfluorodecyltriethoxysilane(PFDS)to obtain the superhydrophobic layer on the MgAl LDH(PEO-LDH-SHS).At last,the perfluoropolyether(PFPE)is dropped into the surface of PEO-LDH-SHS and used to fabricate the slippery surface(PEO-LDH-SLIPS).Evolution of|Z|0.01Hzof each sample over immersion time in 3.5 wt%NaCl solution is shown in Fig.4b.The PEO-LDH coating exhibits a higher|Z|0.01Hzvalue than pristine PEO coating due to the presence of LDH and the.As mentioned above,the LDH serves as the physical protective film to isolate the metal substrate and corrosion solution.serves as the corrosion inhibitors to enhance the corrosion inhibition of LDH.The PEO-LDH-SLIPS possesses high|Z|0.01Hzvalue due to the presence of additional superhydrophobic effect and more slippery surface to inhibit corrosion.From Fig.4c,theis rapidly released from the PEO-LDH coating due to the superhydrophilic structure of LDH.Theconcentration rapidly increases in an early 20 h and reaches about 11.2 mg L-1.An obvious decrease is found in the subsequent period due to the adsorption ofon the surface of Mg alloy.At last,theconcentration maintains stable due to the complete adsorption.However,for PEO-LDH-SHS coating,there are almost noin the solution due to the superhydrophobic effect which inhibits the release offrom the gallery of LDH to corrosionsolution.After the PEO-LDH-SHS specimen is immersed in 3.5 wt%NaCl solution for 15 h,the release process starts.Theconcentration gradually increases to 4.6 mg L-1after 48 h immersion with a slower growth rate when compared with PEO-LDH due to the presence of superhydrophobic coating.Due to the release of,the PEO-LDH exhibits a self-healing effect.After immersed the PEO and PEO-LDH specimen in 0.05 NaCl solution for 16 h,the drilled defect of PEO presents a loose corrosion production(Fig.4d).EDS spectra of PEO specimen immersed in 0.05 NaCl solution for 16 h indicates that the main elements of corrosion production are Mg,O,Al,Cl.However,the drilled defect of PEO-LDH presents a closed interface with main elements of Mg,O,Al,Mo,Cl(Fig.4e).The presence of Mo indicates that theadsorbed on the defect region and formed an insoluble protective film to inhibit the corrosion.In general,the self-healing effect of LDH mainly comes from the sustained release effect of interlayer corrosion inhibitor(Fig.4f).Hence,the release rate of inhibitor anions is particularly important for the self-healing effect.By constructing the coating to delay the release rate of corrosion inhibitor from gallery of LDH,the long-term self-healing of metal can be realized.

Table 1LDH-based systems and measured corrosion resistance properties.(SA:Stearic acid;PPA:Phenylphosphonic acid;AAO:Anodization;La:Laurate;MBT:2-mercaptobenzothiazole;SA-1:Sodium alginate;SDBS:sodium dodecyl benzene sulfonate;NTA:N-alkyl-N,N-dimethyl-N-(3-thienylmethylene)ammonium bromides;ASP:Aspartic acid;PVDF:Polyvinylidene fluoride;PFTS:1H,1H,2H,2H-perfluorodecyltriethoxysilane;FAS-13:Triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane).

Fig.13.(a)The SEM image,(b)partial detail of SEM image and(c)the schematic illustration of ZnAl-VOx LDH/La film[52].

Fig.14.(a)The schematic diagram of the mortar sample,(b)the schematic diagram of the Cl-permeability test in cement mortar;The variation of Cl-binding effect of LDH in concrete with(c)the increase of pH in concrete,(d)the ,(h)the release of the [131].

2.3.Factors affecting the corrosion inhibition

The factors affecting the corrosion inhibition of LDH can be divided into two categories:(1)structural influence factors,(2)own chemical properties.The“structural influence factors”mainly refer to the physical characteristics of the constructed LDH physical protection film,including compactness,porosity and thickness.The essence of the corrosion reaction in the NaCl system is the oxidation and dissolution of metal element [43].The penetration and interface interaction of Cl-with metal atoms is one of the basic conditions for the occurrence of corrosion[44].Hence,sealing the transport channel of Cl-and isolating Cl-from metal substrate are the basic strategies of corrosion inhibition.Modification the compactness,porosity and thickness of the constructed LDH physical protective film is favorable for improving the sealing and isolation ability.To date,the modification of physical properties of LDH film is mainly by adjusting the preparation parameters or adding the additional sealing material[45].The“own chemical properties”mainly refer to the exchangeability of interlayer ions of LDH and derived Cl-binding effect and inhibitor release effect.The exchangeability of interlayer anions endows the LDH unique Cl-binding and nanocontainer effects.The interlayer anions of LDH have a significant effect on anion exchangeability.As mentioned above,theexhibits high affinity with host layer of LDH,leading to difficult intercalation of other anions.Hence,preparation of LDH with exchangeable anion intercalation such asis helpful to enhance the Cl-binding effect.For constructing the nanocontainer of corrosion inhibitor,preparing LDH with exchangeable anion is also needed.The LDH with exchangeable anion can be used as the precursor to prepare the anion-type corrosion inhibitor intercalated LDH by anion exchange[46].Hence,the main factor affecting corrosion inhibitor loading is the interlayer anion exchangeability.The loading capacity of corrosion inhibitor affects the concentration of inhibitor anion in the corrosion solution and the final anticorrosion performance.The release rate of the inhibitor anion affects the release time and the eventual self-healing effect.

3.Synthesis and uses of LDH

3.1.Synthesis of LDH

After years of development,the application of LDH in metal anticorrosion has been greatly developed.Nowadays,various types of LDH have been developed using various approaches including hydrothermal approach,coprecipitation method,memory effect,electrodeposition and MOF conversion,etc.More specifically,the LDH for metal anticorrosion can be synthesized in five different ways,such as in-situ growth,ex-situ growth,memory effect,oxide film transformation and conversion of surface alloy(Fig.5).In the following subsections,each method has been discussed in detail.

3.1.1.In-situ LDH film

●Without metal salts

The host layer of LDH is composed by divalent and trivalent metal cations.Hence,it is necessary to have divalent and trivalent anions in the precursor solution to synthesize LDH.However,some metal alloys such as AZ31 and AZ91 Mg alloy can release divalent and trivalent metal cations in-situ by the oxidation and dissolution of own alloy elements during the synthesis process.This method requires a relatively highspecimens were immersed in the solution and transferred into the reaction chamber.The pressure was maintained at 3 MPa.The reaction temperature was maintained at 50°C for 30 min.Afterwards,the MgAl LDH film was obtained.CO2pressurization facilitates the ionization process of Mg alloy to provide the abundant cation for the growth of LDH.In addition,CO2pressurization accelerated the synthesis process and stability of LDH film.The obtained LDH film exhibits a desirable corrosion inhibition.Delgado et al.[49]studied the influence of Al content of the purchased twin-roll cast AZ31 Mg alloy in the near-surface region on the corrosion rate in 0.6 M NaCl solution.Corrosion results exhibited the Al rich region was more likely to produce the LDH which can block the Cl-transport channel,thereby decreasing the corrosion rate of Mg alloy substrate.Liao et al.[50]studied the corrosion behavior of hot-extruded AZ31B,AM60,AMX602 and AZ91D Mg alloy after exposing 3 years in real environment.All the Mg alloys produced the MgAl LDH phase.The AZ91D alloy possesses optimal durability due to the high mass ratio of Al which is favorable for the formation of LDH phase.

●Adding single metallic salt

Many studies have shown that only one metal salt is needed to synthesize LDH film on the surface of Mg or Al alloy.For examples,Zhang et al.[51]fabricated ZnAl-NO3LDH film on Al substrate by the hydrothermal reaction.Because the Al3+can be dissolved from the Al substrate during the hydrothermal approach,the preparation process does not require the addition of additional Al(NO3)3?9H2O.Cao et al.[52]preparedintercalated ZnAl LDH(ZnAl-LDH-film on Al plates by hydrothermal method using Zn(NO3)2?6H2O and NH4NO3without adding Al(NO3)?9H2O.The obtained ZnAl-LDH-film was used to prepared VOxand laurate intercalated ZnAl LDH by anion exchange.Similarly,Bouali et al.[53]fabricatedintercalated ZnAl LDH(ZnAl LDH-nitrate)film on AA2024-T3 Al alloy by coprecipitation method using Zn(NO3)2?6H2O and NH4NO3without adding Al(NO3)3?9H2O.The obtained ZnAl LDH possesses a morphology of hexagonal nanosheet with excellent crystallinity and protects the Al alloy substrate effectively by its sealing effect.Wang et al.[54]prepared MgAl LDH film on AA6061 Al alloy by hydrothermal process using Mg(NO3)2?6H2O and NaOH without adding Al(NO3)3?9H2O.The element of Al comes from the dissolution of surface Al2O3.Li et al.[55]preparedintercalated MgAl LDH film on AZ31 alloy by hydrothermal approach using Al(NO3)3?9H2O and NaOH without adding Mg(NO3)2?6H2O.The obtained LDH film is used to load the inhibitor(dopamine hydrochloride)by anion exchange so as to enhance the corrosion inhibition performance.

●Adding bimetallic salts

3.1.2.LDH powder

Synthesizing LDH powder directly is helpful to the subsequent chemical modification or coating on metal surface.The chemical properties of LDH are easily modified during synthetic process according the usage requirement.The reaction system is simple and pure,and helps to obtain the pure LDH.Using pure LDH to metal anti-corrosion is helpful to analyze the influence of micro-nano characteristics(including structure,composition,nanometer size and etc.)of LDH on corrosion inhibition performance.As mentioned above,LDH powder can be added to the coating as an additive and act as a nanocontainer of corrosion inhibitor.Of course,LDH-based nanocontainer can be used directly in metal anticorrosion.But corrosion inhibition performance of this method is poor.To date,the method to prepare LDH-based nanocontainer is coprecipitation method.

3.1.3.Memory effect

To eliminate the interference of interlayer ions of LDH,especiallyannealed LDH is used for chloride binding.The LDHs such as MgAl LDH,MgFe LDH and MgCr LDH exhibit the memory effect after calcination at the temperature range from 250 to 450°C[64].The key to memory effect is that the layered structure is not completely destroyed by dehydration process and the coordination form of six coordination for metal cations is not changed to four coordination[65].As shown in Fig.6,Kowalik et al.[66]studied the memory effect of CuZnAl LDH by in-situ XRD and obtained the information of crystal phase transformation with the increase of temperature.The results exhibited that the LDH converted to metal oxides by losing the H2O and CO2at 400°C.The obtained product can be recovered to LDH partly by structural rearrangement under the H2O-containing environment.The LDH converted to metal oxides and spinel at 600°C.The obtained products can not be recovered to LDH due to chaotic structure and variable coordination form.Hence,the calcination temperature has an obvious influence on the memory effect.The recovery of the layered structure from LDH is exothermic process.However,increasing the temperature is conducive to increase the hydroxylation kinetics.Hence,the memory effect away requires temperatures above 100°C.

3.1.4.Oxide film

Converting the oxide film to LDH film has been widely used in corrosion inhibition of Mg alloy and Al alloy.Corrosion inhibition can be achieved by constructing an oxide film to prevent the interface contact between the metal substrate and the corrosion solution.However,the surface of oxide film,which obtained by chemical method,has a large number of holes.Cl-ions can penetrate to the oxide film through these holes and react with the metal substrate to start the corrosion.Hence,converting the film to LDH film is an effective strategy for the corrosion inhibition of Mg or Al alloy.Wu's group developed a complete transformation methods of oxide film to LDH and studied the effects of temperature,time,anion on the obtained LDH phase[28,44,70-73].For example,Zhang et al.[72,74]fabricated an anodic film on AZ31 Mg alloy in a NaOH and NaAlO2solution with 20 V applied voltage for 30 min.Afterwards,the anodic film was converted to MgAl LDH protective film in the NaNO3solution by one-step hydrothermal process.To evaluate the effect of hydrothermal time on the formation of LDH film,different hydrothermal time including 0,10 min,30 min,1 h,2 h,3 h,6 h,12 h were conducted,as shown in Fig.7[44].The pristine anodic film presents an amorphous morphology with large number of pores(Fig.7a and b).When the hydrothermal reaction was performed for 10 min,many nanosheets appeared on the anodic film surface and the pores on the surface were greatly reduced(Fig.7c and d).By prolonging of the hydrothermal reaction,the compactness and crystallinity of LDH film increases,and the pores on the anodic film disappears(Fig.7e-n).The dimension of LDH nanosheets after 12 h hydrothermal reaction is about 0.5-1.0μm(Fig.7o and p).LDH nanosheets with an interlaced morphology are uniformly covered on the specimen surface to achieve the superior sealing effect and effective corrosion inhibition.In addition,they used anodic film to fabricate the MgAl LDH,MgCr LDH and MgFe LDH by hydrothermal process[75].The obtained LDH film,especially the MgAl LDH,exhibits an outstanding sealing effects on anodic oxide film and superior corrosion inhibition.The effect of pH value for the growth of LDH film was also evaluated by Wu et al.[76]The optimal pH for converting anodic film into LDH film is about 10.72-11.72 which has optimal compactness and crystallinity.Similarly,they constructed an LDH film on AZ31 Mg alloy substrate by micro-arc oxidation[71].The micro-arc oxidation was carried out at a stable voltage of 350 V for 10 min using an AC power supply.The 7.14 g L-1NaOH and 4 g L-1NaAlO2were used as the electrolyte.The time required for micro-arc oxidation is shorter than that for anodic oxidation,but the operation is more complex and dangerous.There are also many holes on the surface of the obtained micro-arc oxide film,which is unfavorable to the corrosion inhibition of Mg alloy.Hence,it needs to conduct the physical or chemical modification to seal the pores.The obtained micro-arc oxidation film was used to grow the LDH film in a hydrothermal system at 398 K for 12 h with 0.1 M NaNO3without adding any metal salts.The obtained LDH film possesses a morphology of uniform and compact nanosheets and exhibits an outstanding corrosion inhibition performance.In addition,Zhang et al.[70]prepared LDH film on AZ31 Mg alloy substrate by plasma electrolytic oxidation using pulsed AC power supply with a square electrical signal(100 Hz,+250 V/-50 V,26%)for 600 s.The 7.14 g L-1NaOH and 4 g L-1NaAlO2solutions were used as electrolyte.The as-synthezised oxide film has many pores.The authors used the CeO2which prepared by Ce(NO3)3and H2O2to seal the pores.After that,the oxide film was subjected a hydrothermal reaction to fabricate the LDH layer.The obtained LDH layer was modified byvia a water bath process.The growth and conversion process of oxide film of Al alloy are similar to those of Mg alloy.For example,Liu et al.[77]intercalated NiAlCe LDH film on 2A12 Al alloy substrate by the conversion of oxidation film.The Al alloy specimen was anodized at a stable voltage of 25 V for 45 min using 40 g L-1oxalic acid as the electrolyte.The obtained oxide film was subjected coprecipitation process to obtain the LDH film.The obtained LDH film is the NO3-intercalated ZnAlCe LDH.The author used the Na3VO4?12H2O to prepare theloaded ZnAlCe LDH.The ZnAlCe-VO4LDH sealed the pores of the anodic oxide film on the one hand and enhanced the corrosion resistance on the other hand.In addition,Zhang et al.[78]fabricated a ZnAl LDH film from the chemical conversion of micro-arc oxidation film.The micro-arc oxidation film was constructed using 18 g L-1Na2SiO3+9 g L-1KOH or 18 g L-1NaAlO2+9 g L-1KOH as the electrolyte.The obtained oxide films are named as MAO-Si and MAO-Al.The obtained oxide films were used to grow the LDH film in the solution with Zn(NO3)3?9H2O,NH4NO3.The LDH nanosheets grown on MAO-Al were denser and uniform than that of MAO-Si.The presence of Si particles inhibits the nucleation and growth of LDH.In general,constructing the LDH from the chemical conversion of oxide film is an effective strategy for inhibiting the corrosion of Mg or Al alloy.

3.1.5.Surface alloy

Surface alloying has received wide research interests recently and been considered as the great method to fabricate the compact and robust LDH film to prevent the corrosion of metal substrate.Previously,Song et al.[79]produced the aluminum-alloyed film on AZ91D by the hot-diffusion of Al powder at 420°C.Due to the abundantβ-phase(Mg17Al12)in the obtained aluminum-alloyed film,the AZ91D Mg alloy exhibits superior corrosion resistance and high hardness.Based this result,Zhu et al.[80]fabricated the Zn-Al alloyed surface on the AZ31 Mg alloy by hot-diffusion and dipping process at first,and then obtained the LDH film by the conversion of the surface-alloyed film with NaOH and NaNO3under the hydrothermal process at 393.15 K for 24 h(Fig.8).For steel and its alloy,hot-dip Zn coating offer corrosion resistance for steel due to its physical protection and self-sacrificial effect[81].Since 1960,Zn-Al alloy coating have been widely used in corrosion protection of steel and exhibits an optimized durability.For examples,compared with traditional hot-dip Zn coating,55Al-Zn coating possesses two to four times durability after more than 10 years exposure[82].LeBozec et al.[83]constructed the Zn-Al alloy coating on steel surface by hot dip technique.According to their study,the mass ratio of Al is highly correlated with the durability of Zn-Al alloy coating.With the increase of mass ratio of Al,the corrosion resistance of Zn-Al alloy increases gradually.When the mass ratio of Al reaches the 55 wt%,obvious ZnAl LDH phase obtained after a short time of corrosion.The presence of LDH phase slows the corrosion rate of Zn-Al anodic layer and enhances the overall corrosion inhibition performance of steel component.

Studies in recent years have shown that the corrosion resistance of Zn-Al alloy coating has been further enhanced by adding a certain amount of Mg[84].For example,LeBozec et al.[85]investigated the influence of component and structure of Zn-Al-Mg alloy on the corrosion behavior of steel in seawater.The results revealed that Mg-rich phase dissolved preferentially and then transformed to MgAl LDH on the alloy surface to inhibit the steel corrosion.Duchoslav et al.[86]analyzed the surface chemistry of Zn-Mg-Al coating on steel after a short time of exposure in NaCl solution.With the increase of immersion time,the surface environment of the Zn-Mg-Al alloy was gradually alkalized until the LDH film forms.Subsequently,the LDH layer was found to stabilize the surface pH and hinder the further alkalization process.Hence,the corrosion process of Zn-Mg-Al alloy was inhibited.

3.2.Use of LDH

Currently,many using methods of LDH have been developed:for instance,in-situ LDH film,adding into corrosion solution,incorporated into organic coating or superhydrophobic coating.As mentioned above,LDH usually acts as the physical protective film by the formation of nanoarray to inhibit metal corrosion.There are two types of growth methods,one is the dense packing film,the other is the conversion film.The dense packing film is grown by direct hydrothermal or coprecipitation approach on the surface of metal alloy.The LDH film prepared by these methods generally has adhesion and stability issues.Hence,sufficient aging time for the formation of LDH film is required.For example,Yao et al.[87]prepared MgAl LDH at first by combining the coprecipitation process and hydrothermal process.The coprecipitation process was conducted at 338 K for 48 h following on an aging process for 12 h.The aging process enhances the compactness and stability of the LDH film.

The layered LDH can be used to prepare nanocontainer of anion-type corrosion inhibitor by anion exchange or direct synthesis.LDH-based nanocontainer has been widely used in anti-corrosion field and exhibits the interesting slow-release effect of corrosion inhibitor and Cl-binding effect.Previous studies[88,89]have shown that,corrosion inhibitors have a strong corrosion inhibition effect on metal corrosion by the coordination between heteroatom and metal atom.Hence,due to the presence of corrosion inhibitor,the LDH-based nanocontainer can be added into the corrosion solution directly.For example,Li et al.[60]prepared ZnAl-LDH for the corrosion inhibition of mild steel in 3.5 wt%NaCl solution.Due to the presence ofthe anti-corrosion performance of mild steel is enhanced.Tian et al.[90]preparedintercalated ZnAl LDH.The obtained ZnAl LDH was added into the simulated concrete pore solution and evaluated the corrosion inhibition for carbon steel.served as the corrosion inhibitor and enhanced the corrosion resistance of LDH.The corrosion inhibition efficiency of this method is highly related with the corrosion inhibition performance of corrosion inhibitor.Hence,selecting corrosion inhibitor with stronger corrosion inhibition performance is helpful to enhance the overall corrosion inhibition efficiency of LDH.It is worthy mentioning that the above reports are based on a static system.In a flow system,the corrosion inhibitor released from LDH-based nanocontainer is consume rapidly,leading to a sharply declining corrosion inhibition performance.Hence,adding the nanocontainer into the corrosion solution is beneficial to study the slow release effect and corresponding corrosion inhibition performance,but not beneficial to the practical industrial application.

Fabricating the organic coatings on the surface of metal is regarded as an excellent corrosion inhibition strategy and has been widely used in metal anti-corrosion.Organic coating provides the superior physical barrier and high transfer resistance of electrons and ions,thereby retarding the diffusion of aggressive chemical species and offering the corrosion inhibition.However,the damage or micro-cracks caused by friction or scraping and aging reduce the durability of the coating structure and accelerate the corrosion of the metal substrate.Incorporating the LDH into the organic coating can help to solve these problems.For example,Alibakhshi et al.[91]fabricated the ZnAl LDH/silane coating for corrosion inhibition of mild steel.The ZnAl LDH was used as the nanocontainers of corrosion inhibitors includingandThe LDH-containing silane coatings were sprayed on the mild steel.Theintercalated ZnAl LDH/silane coating exhibits the optimal corrosion inhibition performance due to the superior physical barrier and slow release effect.Subsequently,Alibakhshi et al.[36]preparedintercalated ZnAl LDH using similar method.The obtained LDH powder was added into the silane solution.The silane solution without and with LDH powder were sprayed on the mild steel.As shown in Fig.9c,the ZnAl-PO4LDH/silane coating exhibits the highest corrosion inhibition performance and optimal self-healing property when compared with the pure silane coating(Fig.9a)and ZnAl-NO3LDH/silane coating(Fig.9b),due to the outstanding corrosion inhibition capability ofThe specimen with a scratch has no obvious corrosion after 1000 h exposure in salt spray environment.For comparison,the silane coating and ZnAl-NO3LDH/silane coatings produces the obvious corrosion along with the scratch after 1000 h exposure in corrosion environment.Hayatdavoudi et al.[92]prepared MBT anion intercalated ZnAl LDH.The obtained LDH powders act as the nanocontainers and added into the zinc-rich epoxy(ZRE)coating for effective corrosion inhibition of carbon steel.Based the slow release effect of MBT,the LDH-based coating exhibits a long-term corrosion inhibition.After 25 days immersion in 3.5 wt% NaCl solution,the LDH-based coating still maintained excellent corrosion inhibition efficiency.Zadeh et al.[46]used NaY zeolite as the cationic nanocontainer to load the Ce3+(NaY-Ce)and ZnAl LDH as the anionic nanocontainer to load the MBT(LDH-MBT)The NaY-Ce and LDH-MBT were added into the waterborne epoxy to evaluate the corrosion inhibition performance for AA2024-T3 Al alloy.As the corrosion inhibitors,Ce3+and MBT possess admirable corrosion inhibition for Al substrate.Adding the NaY-Ce and LDH-MBT into the coating promoted the self-repairing effect and improved the long-term corrosion inhibition capability.In general,adding LDH nanocontainer into the coating is helpful to optimize the corrosion resistance of the coating and realize the long-term corrosion inhibition of metal substrate.

In recent years,superhydrophobic coating has been received widely concern in anti-corrosion field.Superhydrophobic coating possesses interesting water-repellent effect,thereby isolating the corrosion solution and exhibiting outstanding anti-corrosion performance[93].Combining the LDH with superhydrophobic coating is helpful to further optimizing the anti-corrosion performance and achieving the high-efficient long-term corrosion inhibition.For example,Liu et al.[94]fabricated an LDH film on stainless steel by combining the oxidation process and hydrothermal conversion process(Fig.10).Afterwards,the stainless-steel mesh which contains LDH layer was modified by the stearic acid.Due to the presence of protective film(LDH)and superhydrophobic film(stearic acid),the specimen exhibits an outstanding corrosion inhibition performance.Kuang et al.[95]fabricated a MgMn LDH on Mg alloy surface and then fabricated a superhydrophobic coating on top of LDH layer using the myristic acid as the hydrophobic reagent.After the hydrophobic modification,the contact angle increased from 46.7°to 154.3°,and the corrosion current density decreased two orders of magnitude.These successful cases demonstrate the effectiveness of LDH/superhydrophobic coatings in corrosion inhibition.

4.Anti-corrosion application of LDH

4.1.Magnesium alloy

Mg alloys,as the lightweight alloys,have been widely used as the structural materials and critical components for engineering application.However,the low corrosion resistance of Mg alloys in corrosion medium,especially in marine environment,limits its extensive application.Fortunately,hydrotalcite phase can be obtained after self-corrosion of Albased Mg alloys and has an excellent corrosion inhibition.For example,Lin et al.[96]synthesized MgAl LDH phase on AZ91D Mg alloy in thesolution by self-corrosion effect.Due to the presence of MgAl LDH phase,the corrosion current density decreased about 70%.Although the corrosion inhibition performance of this method is still not good,it provides useful idea and strategy for the preparation of LDH conversion film on Mg alloy.The preparation technology of LDH has made great progress in the last decade,Nowadays,LDH can be prepared by various method such as hydrothermal approach,coprecipitation method and electrodeposition method.These methods also promote the growth and development of LDH film on the surface of Mg alloy.How to strengthen the stability and compactness is the key issue of directly growing LDH films on the surface of Mg alloys.At present,there are two method to enhance the stability and compactness of LDH film:(1)prolonging the growth time[97]and(2)using the internal elements of the Mg alloy as the source of cations[98].Both methods have their advantages.The suitable growth method can be selected according to the practical demands.In recent years,the growth LDH film through the conversion of oxide film is a popular method.Prof.Wu developed the synthesis methods of LDH film from the conversion of anode oxide film[44],micro-arc oxide film[71],plasma electrolytic oxide film[70].The obtained LDH films by the above methods are compact and stable,which can provide excellent corrosion inhibition capability for Mg alloy substrates.However,the obtained components of the above methods are complex and chemical modification,such as anion exchange,for LDH is difficult.To further improve the corrosion inhibition performance,a superhydrophobic film was constructed on the surface of LDH film using the hydrophobic organic compounds.For example,Wu et al.[99]fabricated an LDH film by the conversion of andic film on the surface of AZ31 Mg alloy.The superhydrophobic films on the surface of LDH layer were fabricated by immersing the LDH film in a stearic acid solution.The Rctvalue of laurate/LDH film achieves 6.25×10-5Ωcm-2and maintains stable after 14 days immersion in 3.5 wt% NaCl.In general,laurate/LDH film exhibits an optimal hydrophobicity and corrosion inhibition ability.Furthermore,fabricating the organic coating with chemical modified LDH on the surface of Mg alloy is another popular research direction in LDH's corrosion inhibition for Mg alloy[41].Organic coating such as epoxy resin,polyurethane has excellent mechanical stability,thermal stability,optical stability and good adhesion,which exhibits the excellent application potential in Mg alloy protection.Self-healing coatings derived from organic coatings have attracted the attention of many researchers.Self-healing effect is achieved by the release of inhibitor anions from the gallery of LDH in organic coating[42,100].In my opinion,the corrosion inhibition ability can be further optimized by combining various corrosion inhibition methods.Lately,

Ding et al.[101]fabricated superhydrophobic coating by three-step operation(Fig.11).First,theintercalated MgAl LDH filmwas constructed on AZ31 surface by combining the coprecipitation and hydrothermal process.Second,the ureido crosslinked polydimethylsiloxane(U-PDMS)layer was fabricated on thelayer to stable the LDH film.Third,the superhydrophobic layer was fabricated on the U-PDMS layer to enhance the corrosion inhibition ability of overall coating.Compared with pure Mg alloy,the final corrosion inhibition performance of the overall coating is improved about 5 orders of magnitude.In Fig.12,current applications of LDH for the corrosion inhibition of Mg alloy have been proposed.Optimizing the anti-corrosion performance of Mg alloy by combining the advantages of various methods is the main development trend of current anti-corrosion strategy.This strategy is named as“hybrid coating”by us.According to Table 1,the corrosion inhibition performance can be improved two to five orders of magnitude by constructing the LDH in-situ film or coating.The result is a great encouragement for researchers to study LDH-based corrosion inhibition methods.

4.2.Aluminum alloy

Earlier,chrome-containing coatings were used to inhibit the corrosion of Al alloys[102],which decreased lately due to the hazardous nature of chromium for human safety and environment.Similar to Mg alloy,LDH is also considered as the effective corrosion inhibitor for Al alloy due to the slow release and Cl-bind effects.In 2003,Buchheit et al.[103]fabricatedintercalated ZnAl LDH.The obtained LDH was used as the nanocontainers to added into the amide-cured bisphenol epoxy resin which then was coated on the 2024-T3 Al alloy.The corrosion inhibition performance of the coating was significantly enhanced due to the presence of LDH phase.In subsequent attempts,various anion-type corrosion inhibitors intercalated LDH were developed and used for corrosion inhibition of Al alloys[104,105].Duan and his group[106]developed a superhydrophobic materials based on LDH and sodium laurate.The LDH layer was fabricated by combining the anodic oxidation of Al substrate and the subsequent hydrothermal reaction.The superhydrophobic layer with a contact angle of 166owas fabricated by immersing the Al substrate with LDH layer into the sodium laurate solution.Although this study did not evaluate the corrosion inhibition performance of the superhydrophobic coating,it provided a practical experience for subsequent studies.In 2008,Duan et al.[107]deposited superhydrophobic LDH film on Al substrate once again and evaluate the corrosion inhibition performance of the obtained coating.The corrosion current density decreased about three orders of magnitude when compared with the untreated Al substrate.In 2009,Duan et al.[108]prepared ZnAl LDH film on Al sheet in-situ.The realization of in-situ growth of LDH on the surface of Al alloy opens the successful door for the protection of Al alloy.After that,growth an LDH protective layer with the intercalation of anion-type inhibitors on Al alloy through chemical modification became a hot research direction.For example,Zhang et al.[109]preparedntercalated ZnAl LDH layer on Al substrate.Subsequently,the anion exchange was conducted to achieve the intercalation of anion-type inhibitors,such asand MBT(2-Mercaptobenzothiazole).The current density ofinteracted LDH achieved 9.44×10-8A cm-2,which is three order of magnitude higher than bare Al alloy.Hence,the hybrid coating exhibits the superior corrosion inhibition performance.Similar to Mg alloys,the protection of Al alloys takes the collaborative way of various corrosion inhibition methods eventually,such as in-situ protection,self-healing,superhydrophobic.As shown in Fig.13,Cao et al.[110]prepared sodium laurate(LA)intercalated ZnAl LDH film on Al substrates by combining hydrothermal method and anion exchange process.The laurate anion intercalated ZnAl LDH has excellent hydrophobicity.The contact angle of the LDH film reaches 152.7°.The corrosion inhibition performance of superhydrophobic film is about 3 orders of magnitude higher than that of pure Al plate.After the specimen immersed in the corrosion medium for a long time,the corrosion inhibition effect of the superhydrophobic film still maintained at a high level and has long-term stability.The hybrid coating provided comprehensive corrosion protection and has full potential as long-time service of Al alloy.

4.3.Steel

Steel is one of the most widely used metal alloys in numerous applications at present owing to its advantages of high strength,outstanding plasticity and excellent heat resistance.However,steel are vulnerable to corrosion in the NaCl-containing environment such as ocean[111].Currently,using the corrosion inhibitor or constructing the organic coating are main strategy for corrosion inhibition of steel.Hence,the application of LDH in the protection of steel also combines the above methods.In 2012,Hang et al.[112]fabricated a 2-Benzothiazolylthio--succinic acid(BTSA)intercalated MgAl LDH/epoxy coating on the surface of steel.Due to the presence of BTSA intercalated MgAl LDH,the corrosion inhibition performance of the epoxy coating increased about two orders of magnitude.Granizo et al.[113]fabricated an organic coating withintercalated ZnAl LDH on the surface of steel.The obtained organic coating presented a low permeability due to the good adhesion with the steel substrate and outstanding corrosion inhibition.There are many other cases in which LDH has been used as a corrosion inhibitor carrier and added into coatings[56,91,92,114].These results show that the corrosion of steel can be effectively suppressed by LDH nanocontainer.Besides,as described earlier,ZnAl LDH phase can be obtained after the corrosion of 55Al-Zn hot dip alloying layer.The obtained LDH phase exhibits desirable corrosion inhibition performance due to the sealing effect[115].Based on this result,the introduction of Mg into the Zn/Al coating can further reduce the corrosion rate due to the concomitant ZnAl LDH and MgAl LDH phase[116,117].The additional MgAl LDH phase optimizes the sealing effect and enhances the corrosion inhibition performance.Unfortunately,the corrosion inhibition efficiency of conversion film of surface alloy is inferior.Developing novel methods to enhance corrosion inhibition performance of LDH for galvanized steel is particularly important.Shkirskry et al.[118]fabricated an organic coating withinhibitor intercalated ZnAl LDH on hot dip galvanized steel.The slow release effect offrom the gallery of LDH improved the long-term anti-corrosion capability of steel.Subsequently,Duong et al.[119]used MgAl LDH to load the benzoate(BZ)anion.The obtained nanocontainer with epoxy coating was coated on the surface of carbon steel.The BZ intercalated LDH exhibited the optimal corrosion inhibition performance.The author proposed that,the dispersion of LDH in organic coatings and the solubility of corrosion inhibitor in corrosion solution are two factors affecting the overall corrosion inhibition performance.It is worth mentioning that,there is no literature report on the in-situ LDH film on the surface of steel.The reason may be ascribed that the weak adhesion between LDH and steel.We look forward to more meaningful work to be reported as soon as possible to enrich the application of LDH in the steel protection field.Moreover,from Table 1,the corrosion inhibition efficiency of LDH-based methods improves 1 to 3 order of magnitude than that of pure steel substrate,indicating the effectiveness of LDH-based coating.

4.4.Cement or concrete

Concrete is one of the basic materials of civil engineering and is often used with rebar to enhance the safety of building structures.Some corrosion properties of concrete related to the cement are summarized in this section.In ocean,concrete is vulnerable to corrosion due to the erosion of chloride ions,thereby endangering the structural safety and durability of concrete.In addition,the carbonization of concrete is also a cause of corrosion of steel reinforcement by damaging the passive film of steel[120,121].Hence,it is important to develop effective methods to inhibit the erosion of chloride ion and carbonation of concrete.Many literatures have shown that LDH can form spontaneously in cement underalkaline conditions[122,123].For carbonization of concrete,the incorporation of LDH into cement also decreases the carbonization resistance by binding the.For example,Ke et al.[124]added calcined hydrotalcite into the slag-based cement.In the curing process,LDH phase was formed from the calcined LDH by the memory effect.The LDH-containing slag pastes exhibits a superior stability.The compressive strength has almost no change even it is exposed to carbonation conditions for 28 days.The reason should be ascribed that,LDH exhibits an outstanding CO2uptake capability.Similarly,Duan et al.[125]studied the effects of LDH on carbonation resistance of sulphoaluminate cement concrete.The carbonate depth was obviously decreased due to the binding effect of LDH on.Hence,the carbonate resistance of concrete is significant enhanced.Besides,Wu et al.[126]reported that the incorporation of calcined LDH does not damage the setting time,hydration degree,microstructure and compressive strength of cement pastes.The added mass of calcined LDH should not be less than 2 wt%.For the erosion of Cl-,Zahid et al.[127]added 2 wt%NO2intercalated LDH into reinforced concrete.Obvious Cl-binding effect is found due to the high Cl-uptake capability of LDH.Yang et al.[128]preparedand pAB(aminobenzonate anions)intercalated CaAl LDH.The two different LDHs were added into the simulated concrete pore solutions with Cl-respectively.Theintercalated LDH exhibits a better binding effect for Cl-when compared with the pAB intercalated LDH due to the rapid exchange kinetic betweenand Cl-.Machner et al.[129]studied the Cl-binding capability of MgAl LDH under the influence of temperature(38°C or 68°C)and salt type(NaCl or CaCl2).Raising the temperature is helpful to enhance the Cl-binding capability of LDH.Compared with NaCl,LDH has a stronger binding capacity for Cl-in CaCl2solution.Yang et al.[130]studied the Cl-and heavy metal cations such as Pb2+,Cu2+and Cr3+binding capability of LDH in the sodium carbonate-activated slag cements.Due to the presence ofin the concrete pore solution,the adsorption active sites of LDH is occupied by thepriority.Hence,the Cl-and heavy metal cations trend to adsorb on the surface of LDH.Hu et al.[27]added the roasted bauxite and MgO into the concrete.The bauxite and MgO were transformed into the MgAl LDH phase via a hydration process.The obtained concrete exhibits a superior Cl-ingress resistance due to the Cl-binding effect of LDH.Concrete is a complex system.To simplify the evaluation process,many researchers evaluated the corrosion inhibition performance of LDH for steel using simulated concrete pore solution.For examples,Tian et al.[90]added theintercalated ZnAl LDH into the simulated concrete pore solution and then evaluated the corresponding corrosion inhibition performance.is a common corrosion inhibitor for steel.The results demonstrated that,intercalated ZnAl LDH exhibited effective long-term corrosion inhibition due to the slow release effect of LDH forWu et al.[68]evaluated the different anions(such asC6H5COO-,Cl-,)intercalated MgAl LDH on the corrosion inhibition performance of carbon steel in simulated concrete pore solution.intercalated MgAl LDH exhibited the optimal corrosion inhibition performance.Xu et al.[59]evaluated the effect ofon the corrosion inhibition of LDH for steel in simulated concrete pore solution.Real concrete can not only existbut also probably a certain amount ofThe results showed that the presence ofreduced the Cl-binding amount of LDH,thereby reducing corrosion inhibition performance of LDH.However,evaluating the corrosion inhibition performance of LDH for rebar in simulated concrete pore solution has obvious problems.It is more accurate to evaluate the corrosion inhibition performance of LDH in real concrete system.Currently,reports on this area are so rare.Fortunately,Cao et al.[131]studied the corrosion inhibition capability ofintercalated MgAl LDH in real concrete system in 2017.intercalated MgAl LDH was prepared by the calcination-rehydration of Mg-Al oxide in NaNO2solution.The corrosion inhibition capability ofintercalated MgAl LDH for rebar in concrete was evaluated by the mold which is shown in Fig.14a.The rebar was embedded into the concrete.A Cu wires was incorporated into the rebar to act as the conductive path.The both ends of rebar was encapsulated by epoxy resin.To evaluate the permeability of Cl-,the mold was designed and shown in Fig.14b.The concrete containing the NO2-intercalated MgAl LDH possesses excellent corrosion inhibition performance.The corrosion inhibition mechanism of LDH for concrete is also analyzed,as shown in Fig.14c-e.Due to the calcination-rehydration,LDH particles have a certain amount of OH-in the gallery of LDH(Fig.14c).When LDH encounters the Cl-,the Cl-ions can be bound into the gallery of LDH with the release of OH-by anion exchange process.Due to the inevitable carbonation process(presence of large amount of),the Cl-adsorption capability of LDH is inhibited.However,due to the presence of Ca2+in the concrete,theis consumed by Ca2+by the formation of CaCO3sediment.Based on this result,the Cl-binding effect of LDH is not appreciably affected(Fig.14d).Importantly,the occurrence ofrelease with the Cl-binding of LDH produces the corrosion inhibition for carbon steel(Fig.14e).Hence,this work provides an important case for the anti-corrosion application of LDH in concrete.In general,the Clbinding effect and nanocontainers of corrosion inhibitors endows the LDH excellent corrosion inhibition for concrete.

5.Conclusions and Perspective

In this review,we summarized the development and application of LDH as corrosion inhibition for metal anti-corrosion.The structural characteristic and corrosion inhibition mechanism of LDH were analyzed at first.The unique layer structure of LDH provides a loading ability for organic or inorganic anion-type inhibitor and can serve as the nanocontainer of corrosion inhibitor to provide the enhanced corrosion resistance and long-term self-healing performance for metal anticorrosion.LDH-based nanocontainer possesses slow release effect of corrosion inhibitor and Cl-binding effect and usually used as the additive to added into organic coating or concrete.The hexagonal nanosheets of LDH can form a physical protective film on the metal surface by dense stacking.The compactness and stability of in-situ LDH film are the key factors affecting the corrosion inhibition performance.At present,the insitu LDH film is widely used in anti-corrosion application of Mg alloy and Al alloy.Superhydrophobic coating possesses interesting water-repellent effect,thereby isolating the corrosion solution and exhibiting outstanding anti-corrosion performance.Researchers have realized that combining the characteristic of various methods is helpful to improve the anti-corrosion efficiency of LDH.As a result,LDH-based hybrid coating is beginning to come into view.Nowadays,LDH exhibits a promising potential for corrosion inhibition of metal or its alloy.Current research has greatly promoted the development and application of LDH.

The conventional approaches to synthesize LDH involves nucleation,growth,agglomeration,and aging stages,which greatly depends on the process parameters such as stirring rate,residence time,reaction temperature,etc.To synthesize high quality LDH,it is necessary to ensure a good control of process parameters.Recently,high-throughput techniques for combinatorial compositional design and rapid synthesis of alloys with corrosion inhibition properties are gaining popularity.Yao et al,synthesized and screened the PtPdRhRuIrFeCoNi compositional space of multilmetallic nanomaterials[140].The similar methodology can also be applied to synthesize high quality LDH as corrosion inhibition materials.Ren et al.[141]reported high-throughput preparation of monodispersed LDH with narrow particle size distribution based on micro reaction technology.A continuous-flow T-type microchannel reactor with a special outlet structure was fabricated and the reaction process was conducted at relatively high Reynolds number.Depending on these conditions,different kinds of LDHs containing binary,ternary,and quadruple metal cations were successfully synthesized.Particularly,MgAl-NO3and MgAl-Cl LDHs were synthesized successfully in the condition without inert gas protection.Beside corrosion inhibition,there are many applications which could arise from the intercalation of key anionic species into LDHs.

However,there are still many problems to be solved:

(1)The Cl-binding effect of LDH is related to the corrosion inhibition capability of LDH.However,have a high affinity with the host layer of LDH.The Cl-binding effect of LDH invalidates easily at the presence of.For example,the large amounts ofin concrete occupies the active site of LDH preferentially and prevent the adsorption of Cl-.exists in the real water,which have an obvious inhibition effect for the adsorption of Cl-by LDH.

(2)Many studies have confirmed that the physical protective film can effectively protect metal.The method has been widely used in Mg or Al alloy by hydrothermal approach,coprecipitation method or conversion of oxidation film.Unfortunately,there is no report about LDH protective film on the steel surface.The reason may be ascribed to the poor interface adhesion between LDH and steel.Hence,LDH is difficult to grow on the steel surface compactly.

(3)LDH is widely used in corrosion protection of Mg and Al alloy,but less used in corrosion protection of steel and concrete.Steel and concrete are important structural materials.Corrosion of steel and concrete causes unbearable economic losses every year.Corrosion inhibition of steel and concrete is related to the safety and stability of many important projects.Therefore,more research efforts about LDH should be pour into the corrosion inhibition of steel and concrete.

In general,as a promising green corrosion inhibitor,LDH can achieve efficient and long-term corrosion inhibition of metal and its alloy.We believe that in comparison to traditional“trial-and-error”experimentation,the combinatorial synthesis approaches in combination with high throughput characterization can accelerate the discovery and development of novel LDH with uniform and high-quality corrosion inhibition characteristics despite compositional complexity.

Declaration of competing interest

No conflicting interest is declared.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the project of the Graduate Research and innovation of Chongqing,China(Grant No.CYB20005),the project of Technological Innovation and Application Development in Chongqing(cstc2019jscxmsxm0378),the National Natural Science Foundation of China(Grant Nos.51908092),the Joint Funds of the National Natural Science Foundation of China-Guangdong(Grant No.U1801254),the project funded by Chongqing Special Postdoctoral Science Foundation(XmT2018043),Natural Science Foundation Project of Chongqing for Post-doctor(cstc2019jcyjbsh0079),Technological projects of Chongqing Municipal Education Commission(KJZDK201800801),Projects(No.2020CDJXZ001,2020CDCGJ006 and 2020CDCGCL004)supported by the Fundamental Research Funds for the Central Universities,the Innovative Research Team of Chongqing(CXTDG201602014)and the Innovative technology of New materials and metallurgy(2019CDXYCL0031).The authors also thank the Electron Microscopy Center,Analytical and Testing Center of Chongqing University for materials characterizations.This work was also supported by Abu Dhabi Award for Research Excellence.